The present invention relates to a high-strength fabric of synthetic fibers or synthetic threads which have both electrically nonconductive threads and electrically conductive threads. The electrically conductive threads preferably contain electrically conductive carbon dispersed in them. The invention relates also to bulk material containers and carrying straps made from this synthetic fiber fabric.
Fabrics of natural or synthetic fibers or threads often tend to take on an electrostatic charge, especially when they are subjected to rubbing at low atmospheric humidity. The tendency to accumulate an electrostatic charge is particularly pronounced in fabrics of hydrophobic fibers; that is, fibers of complete synthetic polymers such as polyamides, polyesters, polyacrylates, polyacrylonitriles and polyolefins.
Electrostatic charges are a nuisance especially on clothing and carpets, since sometimes such charges become so great that a person coming in contact with a grounded object receives a strong electric shock. Moreover, electrostatic charges can be worse than a nuisance when in the vicinity of sensitive electronic circuits. Finally, the electrostatic charges can be very dangerous when such fabrics are used near explosive materials and/or in an environment where there is a risk of explosion.
Bulk containers, made from synthetic fiber fabrics, are used for a great variety of bulk goods. However, an achieved internal resistance in the fabric of the container of no more than 10.sup.4 ohms is still insufficient to overcome the danger of explosion resulting from static charging in the filling or emptying of the containers. In mining, for example, to prevent initiating an explosion due to static electricity produced in the filling or emptying of bulk containers in an area where there is danger of explosion due to gas or vapors, bulk containers have heretofore been used which are made from a fabric incorporating metal threads that dissipate the static charge.
A disadvantage of this solution is that these metal threads are often incorporated into the fabric as warp threads alone, so that their dissipative ability is limited. In addition, the elongation characteristic of the metal fibers or threads differs greatly from that of the rest of the fabric. This often leads to breakage of the metal threads and hence to an interruption of their ability to dissipate static charges. Due to such interruptions, the danger of sparking and explosion are greatly increased if static electric charging takes place.
It is also known to use synthetic fiber fabrics which have been rendered conductive, or not electrically chargeable, by a special chemical sizing to carry off static electricity. It has been found, however, that this antistatic sizing cannot be lastingly applied to the fabric.
The German patent publication DE-B 1,928,330 discloses fabrics which, to prevent electrostatic charging, consist of two different fiber materials. One of these materials contains electrically conductive carbon black dispersed through the entire fiber, while the other is free of carbon black. A disadvantage of this fabric is that, because it contains threads in which the carbon black is dispersed through the entire fiber, if the carbon black is contained in the fiber in an amount sufficient to achieve enough electrical conductivity the strength and stretchability of the fabric are reduced. It is to be noted that sufficient electrical conductivity cannot be achieved if the amount of carbon black contained in the fiber is too small.